The fuel efficiency of modern commercial jet aircraft has become an increasingly important factor in sustaining profitable business operations. Lighter material systems, higher performing and more fuel efficient engines, and improvements in the aerodynamic performance of the aircraft as a whole are three dominant areas that are targeted for increasing fuel efficiency of modern commercial jet aircraft.
Jet engine nacelles are the housings that surround the jet engines on jet aircraft. These nacelles are typically mounted under the wing of the aircraft or on the sides of the fuselage of the aircraft. The nacelle performs a number of important functions such as structurally mounting the jet engine to the aircraft, providing fuel and aircraft system connections to the jet engine, directing air into the engine, reversing engine thrust during aircraft landings, and a host of other critically important functions. As jet engines have grown larger in diameter as a direct result of becoming more fuel efficient, it has become increasingly important that the nacelle which surrounds the jet engine become more aerodynamically efficient to result in less drag and better fuel efficiency.
Given that nacelles on multi-engine jet aircraft comprise a significant percentage of the total frontal cross-sectional area of the entire aircraft, nacelles are one of the best areas for improving aerodynamic performance of the overall aircraft. One of the primary means of improving the aerodynamic performance of the jet engine nacelle is by improving the leading edge surfaces of the nacelle to make these leading edges more aerodynamically efficient. Keeping the airflow attached to the leading edges of the nacelle improves the aerodynamic performance of the nacelle. The further aft on the nacelle the airflow is kept attached, the greater the resulting aerodynamic performance. This attachment of the airflow to the airflow surface is commonly referred to as laminar flow.
As the airflow moves aft on the surfaces of the leading edge of the nacelle, it becomes increasingly important that these surfaces remain completely smooth to facilitate adherence of the airflow to the surface. Minute disruptions in the leading edge surfaces of the nacelle can be caused by such issues as nacelle component split lines, nacelle component steps and gaps, surface irregularities, waviness of the nacelle components, paint lines, and fasteners. These disruptions all have the potential of negatively impacting the desired effect of creating a surface where the airflow will remain attached. Disruptions in airflow along the leading edge of the nacelle results in turbulent airflow that increases aerodynamic drag on the nacelle and on the overall aircraft.
The nacelle component at the leading edge of the jet engine nacelle is commonly referred to as the inlet nose cowl. The inlet nose cowl fulfills a number of functions on the overall nacelle system such as containing a means to keep ice from forming on the leading edge surfaces, providing structural integrity to resist damage from hail, bird strike, and man loading, and to serve as an aesthetically pleasing component on the aircraft. From an engine nacelle aerodynamic performance standpoint, however, the inlet nose cowl fulfills a primary role. To improve the aerodynamic performance of the nacelle, the inlet nose cowl can be designed so that the surfaces take full advantage of laminar flow benefits. One approach includes increasing the forward to aft length of the inlet nose cowl to the practical limits. For example, the inlet nose cowl can be designed so that it extends all the way to the aft bulkhead of the nacelle inlet and thus does not include a mid-span circumferential split line that would otherwise disrupt laminar flow. Furthermore, doors, access panels, and cutouts can be placed aft of a laminar flow area on the inlet nose cowl so that laminar flow is maintained for as great a distance as possible.
While the above-described design features can be achieved with relatively low technical and manufacturing difficulty, fasteners that penetrate the external airflow surface of an inlet nose cowl disrupt the laminar flow. For example, FIG. 1 illustrates a nacelle 100 including an inlet nose cowl 105, and fasteners 110 that penetrate the external airflow surface of the inlet nose cowl 105. The airflow surface of the nacelle 100 also includes other features, such as access panels or doors 115. While the inlet nose cowl 105, and the nacelle 100 in general, may include fasteners in other locations, only a single series of fasteners 110 is illustrated so as not to obscure the present description and illustration of airflow. As the airflow travels from the leading edge of the inlet nose cowl 105, it initially maintains a smooth adherence to the surface of the inlet nose cowl 105. This laminar flow 120, however, is disrupted by the fasteners 110 that penetrate the airflow surface of the inlet nose cowl 105. A turbulent flow 125 continues aft from the series of fasteners 110. The access panels or doors 115, or other features, may also contribute to the turbulent flow 125.
The use of fasteners that penetrate the external airflow surface of the inlet nose cowl persists for a variety of reasons. For example, inlet nose cowls operate in a structurally demanding environment. They must withstand critical design loads while remaining durable and lightweight. This has necessitated and advanced the design of inlet nose cowls that are manufactured from thin and lightweight sheet metal that can achieve acceptable operating results after being mechanically attached to other components such as bulkheads and stiffening frames with fasteners that penetrate the inlet nose cowl and lie, in part, on the external air flow surface. Each fastener, if not installed to the most stringent tolerances, will disrupt laminar flow. It is not uncommon for an inlet nose cowl to have several hundred fasteners, increasing the likelihood that fasteners will disrupt laminar flow.
Attempts to use metal bonding as a replacement for fasteners that penetrate to the external airflow surface of the inlet nose cowl have produced unacceptable results because metal bonding alone has yet to be shown to withstand the high temperatures necessary to keep ice from forming on the inlet nose cowl and the surface area of the bonded interface is insufficient to meet necessary performance requirements. Other methods, such as fusion welding and friction stir welding are plagued with technical difficulties and do not have design allowable values such as flat-wise tension, tensile, fatigue, thermal, corrosive, and so forth to allow an efficient design to be initiated. From a manufacturing standpoint, both of these processes would be unable to fully restore the material temper to an acceptable and homogenous condition, and from a part processing standpoint, the risk of entrapping corrosive chemicals between the welded components during cleaning and the submersed application of protective coatings would be difficult to prevent. Furthermore, the non-destructive methods of inspecting the welded joints are costly and time consuming.